1. Technical Field of the Invention
The present invention relates to yokes for use in rotary electric machines such as, for instance, starter motors, and, more particularly, to a yoke and a method of manufacturing the yoke.
2. Description of the Related Art
In related art, attempts have heretofore been made to provide a method of manufacturing a yoke for use in a starter motor or the like as disclosed in Japanese Patent Provisional Publication No. 64-60247. In the related art shown in FIG. 6, a steel plate 100, cut out in a rectangular shape, is rolled in a cylindrical configuration. During such rolling, a manufacturing method is carried out by causing protrusions 110, formed on one end of the steel plate 100, to be brought into mating engagement with concave portions 120, formed on the other end of the steel plate 100, after which a central area of each protrusion 110 is press caulked in an expanded portion through which the protrusions 110 and the concave portions 120 are coupled to mechanically joint the both ends 100a, 100b of the steel plate 100.
In such a manufacturing method, when rolling the steel plate 100 in the cylindrical configuration, an inlet of each concave portion 120 is formed in a size equal to or slightly less than a width of each protrusion 110 so as to enable the protrusions 110 and the concave portions 120 to be brought into mating engagement with each other in a circumferential direction as shown by an arrow in FIG. 7A. Further, under a condition where the protrusions 110 and the concave portions 120 remain in mating engagement with each other, a clearance 130 is formed between an outer periphery of each protrusion 110 and an inner periphery of each concave portion 120 as shown in FIG. 7B. Subsequently, as shown in FIG. 7C, a central area of each protrusion 110 is press caulked at PW and a peripheral area of each protrusion 110 is expanded to plug the clearance 130 whereby both ends 100a, 100b of the steel plate 100 are mechanically jointed.
However, the related art method encounters various issues listed below.
a) Under a status where the protrusions 110 remain in mating engagement with the concave portions 120, the clearance 130 is formed substantially around each protrusion 110 and, therefore, it becomes hard for the peripheral area of each protrusion 110 to be uniformly expanded upon punch caulking. This results in increased probability where the clearance 130 is left between each protrusion 110 and each concave portion 120 with the resultant occurrence of issues in airtight capability and reduction in mechanical joint strength between each protrusion 110 and each concave portion 120.
b) If a localized expanding portion, formed by punch caulking, is dislocated toward a distal end of each protrusion 110, that is, when a localized expanding portion increases in greater extent toward the distal end of each protrusion 110, a force is exerted to the yoke to cause the protrusions 110 and the concave portions 120 to be drawn apart from each other, causing deteriorated precision in an inner diameter and roundness of the yoke.